NMR study of the role of isopropylsulfates in the two-step “conjunct oligomerization” of propylene and isopentane–propylene alkylation catalyzed by 95% sulfuric acid

Abstract

“Conjunct oligomerization” of propylene or the isopentane‐propylene alkylation catalyzed by an excess of 95% sulfuric acid was performed in two consecutive steps. First di-isopropylsulfate was prepared by interaction of sulfuric acid with propylene. The ester was then either decomposed at room temperature in the presence of the 5‐10 molar excess of 95% acid or was used in the acid-catalyzed alkylation of isopentane. In situ 1 Ha nd 13 C NMR study of the reaction mixture of “conjunct oligomerization” indicated that the diester participates in two equilibria with sulfuric acid. The first one transforms the diester into a monoester. The second equilibrium corresponds to protonation of the monoester with an excess of sulfuric acid. This converts a minor fraction of the mono-alkylsulfate into isopropyl carbenium ions that are only weakly solvated with sulfuric acid: C3H7HSO4 + H2SO4 C3H + H2SO4 + HSO 4 . The subsequent reactions of alkyl carbenium ions with the non-protonated alkylsulfate result in final products of “conjunct oligomerization” while in the presence in the reaction mixture of isopentane, alkylation with the predominant formation of C 8 branched paraffins takes place. A very low yield of propane indicates a minor role of hydride transfer in alkylation. Another unexpected result is the absence in both reaction mixtures of propylene. These findings are in contradiction with the classical mechanism of isoparaffin‐olefin alkylation by Schmerling. Therefore, an alternative mechanism of this reaction is suggested via a direct alkylation of isopentane with the mono-alkylsulfate.